Table 2.

Salient Features and Activities Performed during Specific Time Periods Used in Estimating Exposures for Exposure Groups and Participants in the GuLF STUDY

Time period	Dates	Major activities
1a	April 22-May 15, 2010	Oil was being released at the wellhead near the floor of the Gulf. Efforts to stop the release by containment were unsuccessful. Drilling started on a relief well. Aerial application of dispersant to the water surface began across the Gulf. In situ burns started. Oil reached the LA and MS shore, and beach cleanup started.
1b	May 16-July 15, 2010	Drilling began on a second relief well. Dispersant continued to be applied by plane. Dispersant was injected at the wellhead to break up the oil and applied by ship to the surface of the water near the wellhead to reduce THC air concentrations. In situ burning continued. Offshore skimming of oil and flaring by the rig vessels started. Beach cleanup continued, but now included AL and FL; and onshore and offshore decontamination of vessels and equipment started. On July 15 the well was mechanically capped and the oil flow stopped but the well cap and well casing were still under considerable pressure from the oil formation.
2	July 16-August 10, 2010	The well was “static killed” (stopping the oil flow by pumping drilling mud and other components into the well) on August 10. In situ burning ended by July 19. Offshore skimming ended in early August but nearshore skimming continued. Beach cleanup and decontamination of vessels and equipment continued.
3	August 11-September 30 , 2010	The first relief well intersected the original well about 18 000 feet below the Gulf floor on September 16. Heavy mud and cement were delivered at the base of the well to close off the well. The well was permanently sealed on September 19. Underwater equipment used in the response was removed or repositioned. Large-scale decontamination of the vessels started. Nearshore skimming continued on a limited basis. Beach cleanup continued but started to decline.
4	October 1-December 31, 2010	Decontamination of the vessels and equipment continued. By December 31, essentially all vessels had been decontaminated. Beach and marsh cleanup continued, along with the cleaning of rock jetties, by an increasingly smaller number of people. Nearshore skimming ended in October.
5	January 1-March 31, 2011	Beach cleanup continued by an increasingly smaller number of people.
6	April 1-June 30, 2011	Beach cleanup continued by an increasingly smaller number of people. Distinguished from time period 5 because of the warmer ambient air temperatures, which resulted in higher THC exposure levels.